The present invention relates to carbon nanostructures and methods of growing the same, and more particularly to carbon nanostructures that are attached to catalyst dots, and catalyst-induced methods of growing carbon nanostructures, especially on the tips of cantilevers, nanowires, wafers, conductive micro/nanostructures, and the like.
Programmable Nanometer-Scale Electrolytic Metal Deposition and Depletion
A previous invention, referenced hereinabove, describes nanometer-scale deposition and/or depletion of nanostructures in liquids at preferably ambient temperature and preferably neutral pH through electric field-directed, programmable, pulsed electrolytic metal deposition or depletion.
Application of a programmable and short (nsxe2x80x94ms time scale) pulsing direct current source is used to control the number of atoms being deposited by the electrolytic metal reduction and deposition process. As shown in the following platinum deposition reaction at a cathode using water-soluble hexachloroplatinate, the number of electrons supplied can control the formation of metallic platinum. In electrolytic deposition, electric current and the duration of the current can control the number of electrons.
[PtCl6]2xe2x88x92+4exe2x88x92xe2x86x92Pt↓+6Clxe2x88x92
Other water-soluble metal compounds that have been shown to be applicable include, but are not limited to: PtCl4, OsCl3, Na2[PtCl6], Na2[OsCl6], (NH4)2RuCl6, K3 RuCl6, Na2IrCl6, (NH4)3IrCl6, (NH4)3RhCl6, K2PdCl4, (NH4)2PdCl4, Pd(NH3)4Cl2, ReCl3, NiCl2, CoCl2, PtO2, PtCl2, Pt(NH3)4Cl2, (NH4)6Mo7O24, NaAuCl4, KAu(CN)2K2[PtCl4],and K3Fe(CN)6. Combinations of two or more water-soluble metal compounds can be used sequentially or simultaneously.
As illustrated in FIG. 1, a programmable current source 18 is used to precisely control the number of electrons used to achieve the desired nanometer-scale electrolytic metal deposition. A non-conductive substrate 10 supports nanometer sized electrodes, also called nanowires and nanoelectrodesxe2x80x94cathode 12 and anode 14xe2x80x94which are usually comprised of gold but can be other metals or conductive materials. Spacing between the nanoelectrode tips 13, 15 in the range of 1-10 xcexcm produces results that are suitable for many applications.
A preselected metal 16 is deposited onto the tip of the cathode 12. The metal 16 is usually Pt, but can be any metal that can be deposited electrolytically. A programmable, pulsable current source 18 has electrical connections 20, 22 to the respective nanoelectrodes 12, 14. A bypass circuit 24 is shown, which includes a bypass selector switch 26 and a variable resistor 28.
Nanoelectrodes 12, 14 represent a subset of microscopic sized structures (nanostructures) that are suitable for use. Nanostructures acting as electrodes can be of various sizes and shapes. Spacing between the two nanostructures should not exceed 50 xcexcm, preferably 20 xcexcm, more preferably, 10 xcexcm, most preferably, 5 xcexcm.
The programmable, pulsable current source 18 can be of any suitable construction. Keithley Model 220 programmable current source or the latest Keithley Model 2400 series of Source Meters (available from Keithley Instruments, Inc., 28775 Aurora Road, Cleveland, Ohio 44139, or on the Internet at www.keithley.com) are already capable of supplying a minimum of about 9400 electrons per pulse [500 fAxc3x973 msxc3x97electron/(1.60xc3x9710xe2x88x9219 C)], which could translate to a deposition of 2350 platinum atoms per pulse based on the stoichiometry of the deposition reaction. If this amount of platinum is deposited on the end of a nanowire with a 10 nmxc3x9710 nm cross section, 2350 platinum atoms per pulse can translate into about 1 nm of metal deposition (2.6 layers of platinum atoms) per pulse. The programmable, pulsable current source 18 should be capable of controlling the process so that nanometer metal deposition or depletion as precise as about 1500 metal atoms per pulse can be achieved. A preferable range is contemplated to be 1500xc3x971014 atoms per pulse, although the skilled artisan will recognize that the method can operate well beyond this range.
The bypass circuit 24 is preferably added to fine-tune the electron flow for even more precise control of deposition or depletionxe2x80x94the addition or removal of monolayers or submonolayers of atomsxe2x80x94that can be achieved. The bypass circuit 24 is used to divert some of the electricity away from the nanoelectrodes 12, 14 in order to deposit or deplete fewer metal atoms per pulse. For example, when the impedance of the variable resistor 28 is adjusted to 50% of the impedance between the two nanoelectrodes 12, 14, two thirds of the 9400 electrons per pulse can be drained through the bypass circuit 24. In this case, the electrolytic metal deposition can be controlled to a step as precise as 780 platinum atoms (3130 electrons) per pulse, which can be translated to a deposition of 0.87 layer of platinum atoms 16 on a 10- by 10-nm surface at the tip of the cathodic nanoelectrode 12. By allowing a greater portion of the current to flow through the bypass circuit 24, it is possible to control deposition of metal 16 atoms as precise as 100 atoms per pulse. A preferable range for this extremely finely controlled deposition is contemplated to be 100-2500 atoms per pulse, although the skilled artisan will recognize that the method can operate well beyond this ultrafine deposition range.
The bypass circuit 24 can also protect the nanometer structure from electrostatic damage, especially when the structure is dry. For example, after desired programmable electrolytic metal deposition is achieved as illustrated in FIG. 1, the bypass circuit 24 should remain connected with the nanostructures 12 and 14 while the programmable pulsing current source can then be removed. As long as the bypass circuit remains connected with the nanostructures 12 and 14, any electrostatic charges that might be created during wash and dry of the nanostructures will be able to flow through the bypass circuit 24 that, in this case, comprises the closed switch 26, the variable resistor 28, and wires that connect the switch 26 and the variable resistor 28 with the nanoelectrodes 12, 14. This prevents accumulation of electrostatic charges at any one of electrodes against the other electrode from occurring, thus eliminating the possibility of electrostatic damage at the nanometer gap between the tips 13, 15 of the nanoelectrodes 12, 14.
A special nanostructural arrangement can be used to control the initiation point(s) of nanometer bonding. Special structural arrangements of the nanowire electrodes can be made by various lithographic techniques (e.g., photolithography and electron-beam lithography) to control the initiation point(s) of the electrolytic metal deposition. As shown in FIG. 2, multiple nanowire cathodes 12, 12xe2x80x2 should have respective tips 13, 13xe2x80x2 pointing to the respective tips 15, 15xe2x80x2 of nanowire anode 14 so that the strongest electric field is therebetween. Spacing of the multiple nanowire cathodes 12, 12xe2x80x2 should be regulated to ensure deposition of metal 16, 16xe2x80x2 at the desired cathode location, because the electric field (E) is a vector that is strongly dependent on distance (r):
Exe2x88x9drxe2x88x922
Electrolytic metal-dissolving reactions are applied to deplete metalxe2x80x94open nanometer gaps and control gap size as shown in FIG. 3. By conducting the reversal of the metal deposition reaction with sodium chloride solution instead of hexachloroplatinate as an electrolytic substrate, metallic platinum at the anode tip(s) 16 can be electrolytically depleted via dissolution in a controllable way according to the following reaction:
Pt+6Clxe2x88x92xe2x86x92[PtCl6]2xe2x88x92+4exe2x88x92
This metal-dissolution reaction should also be able to control the gap size between the nanoelectrode tips 13, 15. The site and the extent of electrolytic metal depletion can also be controlled by proper selection of the desired polarity of the electric field and by use of a programmable current source with a bypass circuit, as described herein.
The salient features of the method described hereinabove may be applied in full, in part, or in any combination. Any number of nanostructures can be simultaneously bonded or dissolved on a particular substrate.
For metal deposition, the nanostructure to be metal-deposited does not have to be metal. The method can be used to connect any conductive nanowires such as, for example, nanotubes, especially carbon nanotubes, because of their capability for nanometer electrolytic metal deposition.
For metal depletion, the nonmetallic ions do not have to be Clxe2x88x92. Any anions, such as Fxe2x88x92 and CNxe2x88x92, that can electrolytically dissolve metals (Pt, Pd, Au, etc.) may be used as alternative versions of the method.
Carbon Nanotubes/Fibers and Catalyst-induced Growth on 2-D Surfaces
Patterned growth of individual and mutiple vertical aligned carbon nanotubes and fibers has been experimentally demonstrated on a nanocatalyst-doped two-dimensional surface. See Merkulov, V. I., D. H. Lowndes, Y. Y. Wei, G. Eres, and E. Voelkl (2000) xe2x80x9cPatterned growth of individual and multiple vertically-aligned carbon nanotubes,xe2x80x9d Appl. Phys. Lett. 76, 3555.
Carbon nanotubes possess a number of unique properties, some of which make carbon nanotubes ideally suited for use as probe tips in scanning probe microscopy (SPM). Firstly, single-wall carbon nanotubes have intrinsically small diameters (xcx9c1.4 nm), which allow significant improvement of lateral resolution compared to conventional Si, SiN or other tips, which typically have diameters of at least 10-20 nm. Secondly, carbon nanotubes have very high aspect ratios, AR (AR=length/diameter), which provides the ability to measure deep and near-vertical features, especially those having a sidewall slope that cannot be reproduced accurately using conventional tips. Thirdly, carbon nanotubes exhibit high mechanical strength and flexibility and therefore will not break easily upon crashing into a sample surface, which is a fairly common accident in SPM. Fourthly, some carbon nanotubes are electrically conducting, which permits their use for high-resolution scanning tunneling microscopy (STM). Finally, carbon nanotubes can be chemically selectively modified by attaching organic (or other) molecules at their ends, which creates the possibility of using them as functional probes to detect a particular property or molecule of interest.
Multi-wall carbon nanotubes are somewhat larger in diameter than single-wall carbon nanotubes but they share many of the same properties, to varying degrees. Single-wall carbon nanotubes and multi-wall carbon nanotubes differ primarily in their diameter and morphology. A single-wall carbon nanotube can be thought of as a one-atom-thick sheet of carbon atoms, arranged in the hexagonal graphitic crystal structure, that is rolled up and its edges joined seamlessly (edge atoms overlapping) to form a cylinder. A multi-wall carbon nanotube consists of two or more such concentric cylinders.
Closely related to carbon nanotubes are carbon nanofibers. Carbon nanofibers differ from single-wall carbon nanotubes and multi-wall carbon nanotubes mainly in their crystalline perfection, i.e. the graphite lattice contains many structural defects so that different layers (of a multi-walled fiber) can be either interrupted or joined to one another (or both types of defect can occur at different locations along the length or around the periphery of the fiber). One consequence of these defects is that the electrical properties of carbon nanofibers are not as good as for carbon nanotubes. Carbon nanofibers are synthesized at somewhat lower temperatures than carbon nanotubes and their lower growth temperature is responsible in part for these defects. However, we have recently demonstrated that carbon nanofibers also can be grown with sufficiently small diameter to be of interest for scanning probes. Furthermore, there may be future situations for which a low growth temperature is desirable, e.g. sufficiently low-temperature growth will be compatible with on-board electronic circuitry.
The advantages of carbon nanotubes as scanning probes have been demonstrated by placing carbon nanotubes at the end of a conventional Si tip. The carbon nanotubes were either attached to the tip by electrical and mechanical means in an optical or scanning electron microscope or were grown directly on a tip using conventional high-temperature chemical vapor deposition (CVD) along with catalytic pore formation. However, these procedures have a number of disadvantages: (a) the carbon nanotubes first must be grown at high temperatures and separated, cleaned and cut to length; (b) in most cases it is difficult to adjust the length of the nanotube tip and to obtain a strong, reliable attachment; and (c) these complicated manipulations of carbon nanotubes are impractical in an industrial environment or for large-scale production. See the following: Dai, H., J. H. Hafner, A. G. Rinzler, D. T. Colbert, and R. E. Smalley, Nature 384, 147 (1996); Nishijima, H., S. Kamo, Seiji Akita, Y. Nakayama, K. I. Hohmura, S. H. Yoshimura, and K. Takeyasu, (1999) Appl. Phys. Lett. 74, 4061; and Hafner, J. H., C. Li Cheung, and C. M. Lieber (1999) Nature 398, 761.
Accordingly, objects of the present invention include: provision of methods of fabricating carbon nanostructures, especially for use as scanning probe tips; methods of growing perpendicularly oriented carbon nanostructures directly on the tips of scanning probe cantilevers, nanowires, conductive micro/nanostructures, wafer substrates and the like, particularly with precise control of crucial tip parameters (shape, position and length) and mechanically very strong connections to the substrates. Further and other objects of the present invention will become apparent from the description contained herein.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a method growing a carbon nanostructure, which includes the steps of:
a. providing a substrate;
b. depositing a catalyst dot onto the substrate; and
c. growing a carbon nanostructure on the catalyst dot.
In accordance with another aspect of the present invention, an article comprising a substrate having an adherent metal dot disposed thereon, the metal dot having a carbon nanostructure disposed thereon.